Well, in the absence of other replies, as one who shoots with both Canon and Nikon, I will mention that I use a Canon EF 100mm 2.8L Macro with Image Stabilization, and a Canon Macro Ringlite, to photograph injuries inside the mouth, using a Canon 7D camera. The ring flash attached to the objective end of the lens, and puts light where it is needed, inside the mouth, or other similar space. Of course, a Canon Macro Ringlite is not what you need for your Nikon camera.

The manufacturer of the equipment does not matter, so much as how it is used. Nikon makes an 85mm macro/micro lens for DX cameras, such as yours, and a 105mm macro/micro that works with both DX and FX Nikons. Either should work about as well as my Canon 100mm. The 40mm and 60mm Nikkor options might require getting a bit too close for comfort. (Nikon used the term "micro" in the way that most use "macro.")

There are third-party lens options. I have experience with the Tokina 100mm 2.8 AT-X macro lens on Canon cameras, and it is an excellent value, also made for the Nikon mount. I really like the Tokina for shooting flowers with my Canon 5D, while my Canon macro lens virtually lives on one of my 7D bodies. Not all third-party macro lenses may be so easily paired with all specialized macro flash units, so keep that in mind.

I do not think Nikon makes a ring flash, but does make small a quite expensive kit that combines an SU-800 commander unit with tiny flash units that affix to the objective end of the lens. An alternative would be to see if a third-party flash manufacturer, such as the well-regarded Nissin, makes a ring flash compatible with Nikon. (I seem to recall Nissin making such a ring flash.)

Nikon did make ring flashes in the past, during the film era. I think the SB-29s, the most recent, is compatible with Nikon DSLRs, but will not work with present-day I-TTL metering, so one must set it manually.

Actually, a normal SB-series Speedlight can be used for this type of shooting, by using the bounce card to direct light into the mouth. There will probably be some shadows, that a specialized macro flash would be able to eliminate, but the effect may suffice for your needs, if you have such a Speedlight and wish to try it before buying a specialized macro flash.

To be clear, I have little experience shooting macro with Nikon equipment, and it was with a mentor's employer's equipment, several years go, photographing fingerprints on glass windows. We used a normal SB-800 Speedlight. (evidentiary photography)

When I was at the dentist, they used the AF-S 105mm VR macro lens, with a ringflash, possibly an SB-29. The 105mm focal length will help you keep a slightly more comfortable distance to your subject. But if budget is a concern, Tamron makes a great 90mm macro as well.